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During the peak of last summer we had several goldfish that died after several days of hot weather and I suspect it was related. According to the Goldfish article on Wikipedia:

Extremely high temperatures (over 30°C (86°F) can also harm goldfish

The ambient temperature sat over over 40°C (104°F) during the day and only dropped to a little under 30°C (86°F) overnight so it's almost certain the tank temperature would have exceeded that amount.

Doing some research I see there are some commercial tank cooling products available although they are relatively large / expensive and this is only a once or twice a year event in my area. A few thoughts I'd considered are:

  • Setup a fan pointing across the tank as a form of evaporative cooling. I assume though that would cool the top layer of water more than the bottom and I wasn't sure if the differential in temperature may make things even worse?

  • Introduce a large amount of tap water that I measured as a bit under 20°C (68°F) because it comes from a cooler mountain range. Considering that would need to be treated quickly chemically I could see that also having potential downsides.

  • Add ice cubes that have already been treated, but I imagine that could result in them floating on top and mainly cooling the top portion of the tank.

I wondered if anyone has been in a similar position and can recommend a good alternative or suggest expert opinion on which of the above options may be best?

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    Switching off the lights should help mitigating the problem as they probably introduce a reasonable amount of additional heat. But that's just a guess and thus only a comment. – Baarn Oct 9 '13 at 13:04
  • @Baarn, thanks for the suggestion but they were off at the time, we only use the lighting during winter when the sunlight hours are less. – PeterJ Oct 9 '13 at 13:08
  • There are home aquarium chillers available, but they're pretty expensive. A google search found a YouTube video of a guy who did a custom one using about $30 worth of parts. – John Cavan Oct 9 '13 at 18:55
  • Pretty sure constant temperatures over 104 F / 40 C are bad for YOU too. Gosh, at this point I think improving your own conditions might take precedence, here. – rlb.usa Jul 10 '15 at 21:14
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Well, you didn't say so, but I assume you have a thermometer inside your tank, don't you? Or did you measure the temperature using a portable, temporary one? Since your tank is inside your house, it's not directly at sunlight, perhaps some breeze was on it, etc.

For the solutions, since it only happens few times a year, I'd go with ice cubes of pre-treated water, or as Baarn suggested, water that you have taken from your aquarium (although it might get... smelly... when you freeze and unfreeze it, since you´ll be killing some bacteria). Since they won't have chemicals that are harmful for your fish, it can be used without problems.

Since some energy input is required to transform ice to water, the ice will "steal" this energy from the water, cooling all the tank. And due to convection, the cold water around the ice cube will flow to the bottom of the tank, and the warmer water will flow to the top, get in contact with the ice cube, and doing a cycle.

You'd just need to make things slow (so you won't be changing the water temperature too fast, causing some cold shock on your fish). The thermometer will be your friend here.

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    Pre-treated water sounds complicated, I would use water from the aquarium. – Baarn Oct 9 '13 at 19:08
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    Thanks very much, I hadn't thought of the obvious convection effect and have a large freezer so will go with the ice. The tank doesn't have a thermometer at the moment but I have one I can easily mount to it and will take care it doesn't drop too fast. – PeterJ Oct 10 '13 at 7:28
  • You do not need to use special water, if you can remove the water after melting. For example a simple bottle would help here, or some kind of bag, where the ice-cubes are in. There are also re-usable ice cubes, but them are a more expensive solution – Allerleirauh May 5 at 4:58
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I have had the same issue last couple of years. I thought instead of making icecubes and treating etc also adding more water to the tank, I filled three 2ltr water-bottles and froze. No worries for the smell. Also using one at a time in the tank allowed me to have a standby ready anytime I needed. Seemed to do the trick. I lost no fish. I should say though that I have goldfish so are quite hardy to cold not so much to heat.

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Ice cubes are a good solution. I live in India where the temperatures go above 40 degrees Celsius for about 4 months of the year. Ice Cubes are what keep my fish alive. During these months my fishes enjoy the ice cubes - I have often found then fiddling around and playing with the ice cubes.

Make sure not to add more than 1% of the aquarium capacity in ice at a time as this could disturb the fish. Add ice cubes 3 times a day every 2-3 hours.

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One solution I saw recently was ice, but rather than mixing it with the tank water, you add icecubes inside a sealed plastic bag like a sandwich bag.

This drives down the temperature while the ice melts, but doesn't introduce anything unexpected - our local water supply has become chlorinated recently, and its quite disgusting.

Another useful point is the little air in the bag keeps it floating, which is easier to fish out, and you can re-freeze it well enough if you're mindful of leaks.

Bonus tip - add a water thermometer inside the bag so you can see when its no longer effectively cooling and due for replacement.

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The problem with high temperatures is that it reduces the content of dissolved oxygen in the water. Just add aeration and/or water movement (to increase the surface area of air-water interface). I have a small pond with a few 30 inch (76 cm) koi: the temperature has reached 85 °F (29 °C), I seldom check it. This is no problem with circulation.

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Setup a fan pointing across the tank as a form of evaporative cooling. I assume though that would cool the top layer of water more than the bottom and I wasn't sure if the differential in temperature may make things even worse?

A fan is a fantastic method for lowering a tank temperature efficiently; when researching methods to effectively cool a 60 gallon axolotl tank, I found several recommendations to use a fan. A small, two-head aquarium fan resting in the gap between the back of the hood and the back of the tank has been sufficient to reduce the temperature of that tank 10-15 degrees F or 6-8 degrees C. As the resident is an axolotl, a species capable of launching themselves out of their tanks, the openings in the hood are minimized and kept small enough to prevent her from fitting through them; even with this limitation, the tank stays consistently in the low 60s °F (16-20 °C), as measured from the bottom of the tank. As in other methods offered, convection carries the cold water from the surface down into the tank, allowing warmer water to rise back up to be cooled.

There are some caveats you'll want to consider with this process. You'll need to keep a thermometer in the tank (you should use one anyway). Because this method of cooling is much more effective than you may think, you'll want to keep an eye on it and make sure the tank isn't overly chilled, as well as be able to make adjustments to keep the temperature stable. Most aquarium fans have adjustable heads, which allows the direction of the fan to be modified; some may have separate controls for each head, or multiple speeds as well.

Secondly, because you're using evaporative cooling, your water level is going to drop much more quickly than it would without the fan. I do a weekly partial change and refill on my tank because of how far the water drops; if I let it go too much longer than that, the water level can fall below the upper inlet on the filter, causing the filter to stop drawing in water (and therefore stop working). Depending on your current tank maintenance schedule, you may need to perform it more frequently when using a fan for cooling.

If you need to manage cooling a tank for an extended period of time, a fan may be a more effective way of handling it than ice cubes, requiring less frequent maintenance and producing a more stable temperature result. Aquarium fans are generally rather inexpensive and can be easily acquired online. That said, if the period of cooling is only a day or two at a time, ice is effectively "free" compared to the fan and the time spent managing the tank temperature may be balanced by the reduced time doing overall tank maintenance and refilling it. For my own purposes, requiring the tank always sit below 65 °F / 18 °C (with ambient room temperatures around 70-75 °F / 21-24 °C), a fan is a much better option for me than using more temporary measures; if your tank is generally at the right temperature with only occasional spikes, then you may find less benefit from constant cooling.

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A friend of mine uses the water bottles. He also put Styrofoam or bubble wrap around to insulate the tank from the heat. I used Styrofoam around my tank when we had a power outage in January. The tank was not in the room with the fireplace.

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    SAFETY HAZZARD Making ice cubes out of fish tank water??? An unsuspecting person grabs a cube for their glass of pop. – Bruce Bedford Sep 7 '17 at 17:35

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